Many olefin polymerization catalysts are known, including conventional Ziegler-Natta catalysts. While these catalysts are inexpensive and can provide a broad molecular weight distribution, they exhibit low activity and are generally poor at incorporating α-olefin comonomers. The large variety of active sites in Ziegler-Natta catalysts makes it difficult to control polymer architecture. To improve polymer properties, single-site catalysts, in particular metallocenes are beginning to replace Ziegler-Natta catalysts. Often, single site catalysts provide polyethylene with narrow molecular weight distribution.
Slurry reactors are in widespread use for production of polyethylene homo- and copolymers. Slurry reactors include stirred-tank reactors and water-jacketed tubular reactors arranged in a series of continuous horizontal or vertical loops. A “slurry solvent” in which polyethylene has low solubility constitutes the continuous phase in such reactors. The slurry is intensely stirred in a continuous stirred-tank reactor or series of reactors or, in the case of slurry loop reactors, is driven around the loop at relatively high speed by one or more rather massive pumps. Ethylene, supported catalyst, comonomers, and processing additives are injected into the reactor where polymerization takes place, creating a slurry of polyethylene in solvent.
U.S. Pat. Nos. 6,232,260 and 6,451,724 disclose the use of transition metal catalysts based upon indenoindolyl ligands. Indenoindolyl catalysts are remarkably versatile because substituent effects and bridging changes can often be exploited to provide polymers with tailored physical or mechanical properties. Unbridged indenoindolyl complexes (as exemplified in the '260 patent) usually provide favorable activity although they sometimes fail to provide polymers having high enough molecular weights. Bridged indenoindolyl complexes (as taught, e.g., in U.S. Pat. No. 6,908,972) readily copolymerize α-olefins and provide polymers with low densities. In the '972 patent, several bridged complexes are exemplified in a slurry process and the molecular weight distribution ranges from 2.66 to 7.20. U.S. Pat. No. 6,995,216 uses a silica-supported dimethylsilyl-bridged indenoindolyl complex with hydrogen and 1-butene in a two-stage slurry ethylene polymerization process. The molecular weight distribution is 3.7.
Despite the considerable experience with single-site catalysts generally and indenoindolyl catalysts in particular, there is a need for improvement. Many single-site catalysts produce polyethylene with narrow molecular weight distribution. For many applications, a broad molecular weight distribution is often required. One solution to this has been the use of mixed catalysts or blending of polyethylenes of two different molecular weight distributions. While these techniques provide broad molecular weight distribution, they increase process complexity and cost. Some catalysts, such as Ziegler-Natta catalysts, provide broad molecular weight distribution, but they have lower activity and poor comonomer incorporation. Catalyst activity is important because at low activity, the cost increases as does the potential for undesirable effects of the residual catalyst in the final polyethylene. Generally, when one needs broad molecular weight distribution, a compromise is made. A valuable process would enable high activity, broad molecular weight distribution, and preferably good comonomer incorporation from a single catalyst.